Brian Moore wrote on a refereeing forum a few years ago about managing the scrum. He has a lot of opinions on the matter. He also knows what he's talking about. Somebody compiled the things he said into a post on RugbyRefs.com. I've reprinted the text below.
Some of the information is a little dated because the laws have changed, but the principles remain the same.
Managing the Scrum – Tips by Brian Moore (ex England International Rugby Hooker)Moore represented England, winning a total of 64 England caps between 1987 and 1995, making him the 13th most-capped Englishman (as of July 2007). Moore played in three Rugby World Cups including in 1991 where along with Jason Leonard and Jeff Probyn he was part of a destructive English front row as they reached the final, losing a tight match 12-6 to Australia at Twickenham. Moore was also a member of the England side which won Grand Slams in 1991, 1992 and 1995. In 1991, he was voted Rugby World Player of the Year, a decade before the sport's governing body (the IRB), began its awards programme. He went on two British and Irish Lions tours, winning five test caps. In Australia in 1989, the Lions won the series 2-1, and Moore was famously caught celebrating the morning after on Sydney Harbour Bridge, doing aeroplane impressions. Having been a vocal critic of referees for many years. Moore took the Rugby Football Union’s Entry Level Referee Award course and qualified as a referee in 2010. (http://en.wikipedia.org/wiki/Brian_Moore_(rugby_union)) The following are tips by Brian Moore in managing the scrum, these tips were extracted from a rugby refereeing forum and was submitted by Brian under the alias/user BCM666. In this forum BCM666 (Brian) says “anything I post can be disseminated to anyone; including Paddy O'Brien!” so it is with this general exclaimer that the following document is produced.Pre-game brief about scrum
opposition have pushed over it before the ball is put in.
This is an example that is often put to me - there is no definitive answer to this but you should consider the following: